The present invention generally relates to a clock generating method and a clock generating circuit. More specifically, the present invention is directed to a circuit technique suitably utilized in an EMI (Electro-Magnetic Interference) technique in the case that clocks are generated by a PLL (Phase-Locked Loop) circuit.
In general, a spectrum spread generating circuit for slightly spreading a spectrum of an oscillating frequency so as to reduce a peak of unwanted electromagnetic radiation is arranged by a PLL (Phase-Locked Loop) circuit and a modulator. The PLL circuit is constituted by a phase frequency comparator, a charge pump, a loop filter, a voltage-controlled oscillator, and a feedback-purpose N frequency divider. While a program counter function capable of changing a frequency dividing ratio is given to the feedback-purpose N frequency divider, the frequency dividing ratio is changed in accordance with a modulation data produced based on a modulation profile generated by the modulator, so that the PLL circuit can realize a spectrum spread. In such a spectrum spread clock oscillator, a modulation profile for performing a frequency modulation is converted by a single dimension (both amplitude and time period are constant) by the modulator, and then, is repeatedly frequency-modulated so as to spread the spectrum. As examples of such a clock generating circuit, U.S. Pat. No. 5,488,627 (corresponding JP-A-7-235862), JP-A-2001-202153, JP-A-2004-104655, and the like have been proposed.
Among systems for realizing spectrum spread operations, in particular, in a spectrum spread clock oscillator in which modulation data produced by a modulator is added to a program counter capable of changing a frequency dividing ratio of a frequency divider arranged in a feedback loop so as to be frequency-modulated, a modulation frequency component corresponding to a modulation profile passes through a loop filter (namely, low-pass filter) of a PLL circuit. As a result, in such a loop filter designed in a band where a higher harmonic component of a modulation frequency cannot pass therethrough, the frequency modulation cannot follow the frequency modulation profile at a turning point and is stagnated, and thus, peaks are produced at both edges of the spectrum waveform after spread spectrum modulation, so that the effect of modulation of the spread spectrum modulation is reduced. Also, in the above-described U.S. Pat. No. 5,488,627, the modulation profile called as “Hershey Kiss” corresponds to such a technical idea that in the waveform where the frequency modulating speed near the turning point is increased, the time duration for which the frequency modulation is stagnated is shortened so as to reduce the peaks produced at both the edges of the spectrum, by which the gentle peak is produced at the center portion, so that the peaks of the overall spectrum are lowered. However, the specific loop filter capable of penetrating therethrough this modulation profile, containing the PLL circuit arranged at the post stage thereof, must be designed.